So how/when does the power come back on? If you suspect you might have a bad shore power cord, when you get to the boat and find it has no power, you can use a pen type current tester and run it down the outside of the cord, if there is a break in the hot wire of the cord you will lose power at some point along the cord and that is where the break is.... If you have power all the way to the boats shore power connector then you will need to look at the wiring inside the boat. Of you can get to the backside of that connector inside the boat, you can use that same tester to see if the power is transferring through the shore power connector, if so move to the breaker panel.... etc...etc, but this has to be done when the problem is happening.... It sounds to me like you have a connection somewhere that might be heating up. Good luck, hope you find your problem.